An analysis of the climate change effects on pesticide vapor drift from ground-based pesticide applications to cotton

Vapor drift of applied pesticides is an increasing concern. Among the major crops cultivated in the Lower Mississippi Delta (LMD), cotton receives most of the pesticides. An investigation was carried out to determine the likely changes in pesticide vapor drift (PVD) as a result of climate change that occurred during the cotton growing season in LMD. This will help to better understand the consequences and prepare for the future climate. Pesticide vapor drift is a two-step process: (a) volatilization of the applied pesticide to vapors and (b) mixing of the vapors with the atmosphere and their transport in the downwind direction. This study dealt with the volatilization part alone. Daily values of maximum and minimum air temperature, averages of relative humidity, wind speed, wet bulb depression and vapor pressure deficit for 56 years from 1959 to 2014 were used for the trend analysis. Wet bulb depression (WBD), indicative of evaporation potential, and vapor pressure deficit (VPD), indicative of the capacity of atmospheric air to accept vapors, were estimated using air temperature and relative humidity (RH). The calendar year weather dataset was trimmed to the cotton growing season based on the results of a precalibrated RZWQM for LMD. The modified Mann Kendall test, Pettitt test and Sen’s slope were included in the trend analysis suite using ‘R’. The likely changes in volatilization/PVD under climate change were estimated as (a) average qualitative change in PVD for the entire growing season and (b) quantitative changes in PVD at different pesticide application periods during the cotton growing season. Our analysis showed marginal to moderate increases in PVD during most parts of the cotton growing season as a result of climate change patterns of air temperature and RH during the cotton growing season in LMD. Estimated increased volatilization of the postemergent herbicide S-metolachlor application during the middle of July appears to be a concern in the last 20 years that exhibits climate alteration.

In the United States, the Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA) 1 is the federal statute that governs the registration, distribution, sale, and use of pesticides. Before a pesticide can be sold or distributed, it must be registered/licensed with the United States Environmental Protection Agency (USEPA) under FIFRA. For registering a pesticide with the USEPA, the applicant must show that using the pesticide according to the label directions will not generally cause unreasonable adverse effects to human health or the environment. To enforce FIFRA requirements, the USEPA conducts inspections, surveillance, and pesticide sampling and analysis. Assessment of inhalation exposure from pesticide volatilization is a part of the USEPA's registration and review process. A state may regulate any registered pesticide within the state except those prohibited by the FIFRA. The state can also register additional uses of a federally registered pesticide. However, the state will not impose any requirements for labeling/packaging in addition to those imposed by FIFRA. Off-target drift is one of the adverse effects of pesticide application causing concern to the health of humans and sensitive plants and animals.
Off-target drift and any other pesticide-related environmental concerns can be reported to the state pesticide regulatory agency (for Mississippi, the Mississippi Department of Agriculture & Commerce Bureau of Plant Industry, Pesticide Program), a local agency such as a county extension office or to the USEPA directly. The other source of reporting can be the pesticide manufacturer 2 . The report should be accompanied by clear evidence such as photographs or samples of damaged foliage, logs of weather (temperature, relative humidity and wind speed), spray application details of nearby farmers, and statements from witnesses. Additionally, continuous Methods Study area. The study was conducted in one of the important cotton production regions in the country located in Stoneville, MS, a part of Washington County and the LMD region (Fig. 1). Although the area receives plenty of precipitation, some irrigation is required to supplement highly variable precipitation events during crop growth. Fertile soil (Dubbs silt loam: (fine-silty, mixed, thermic Typic Hapludalfs)) and adequate sunshine generally support successful crop production. Soybeans, cotton, and corn are the major crops in the region. However, other crops are also cultivated. The majority of pesticide use in the region is for cotton. Therefore, pesticide applications during cotton production were chosen for analysis. This study was focused on understanding how climate change occurred in the region and how it could have impacted pesticide vapor drift in the region; therefore, this work is unique from many other studies.

Estimation of wet bulb depression (WBD).
In this study, a simple weather-based parameter, namely, wet bulb depression, which is indicative of evaporation potential, is used to analyze the potential changes in pesticide vapor drift arising from the climate change that occurred in the region. Wet bulb depression is the difference between the observed air temperature (or dry bulb temperature) and the wet bulb temperature. The wet bulb temperature was estimated from the dry bulb temperature and relative humidity using the Stull formula (Eq. 2) as described by the omni-calculator 25 . Equation (2) is applicable for relative humidity between 5 and 99% and temperatures between − 20 and 50 °C.
where T is the dry bulb temperature (°C) of air (or air temperature) and T w is the wet bulb temperature (°C).
where T w is the wet bulb temperature (°C), T is the dry bulb temperature (normal air temperature) (°C), rh is relative humidity (%).
Estimation of vapor pressure deficit (VPD). The vapor pressure deficit between the evaporating surface and air indicates the capacity of the space available in the air for accepting vapors from the evaporating surface.
In the case of pesticides applied to soil/plant, more VPD is indicative of more volatilization from the applied surface. VPD is estimated based on the following equations: where VP EvapSurf is the vapor pressure of the evaporating surface (leaf/soil) in kPa, VP air is the vapor pressure of air in kPa. Vapor pressure can be estimated using the Tetens 26,27 equation (Eq. 3):  www.nature.com/scientificreports/ where T is the temperature of the evaporating surface in °C. For our analysis, long-term records of leaf/soil temperatures are not available. Therefore, air temperature is used for computations. However, in general, plants without water stress will have leaf temperatures cooler than the air temperature. Because we use air temperature in Eq. (3), the result is the same as the saturation vapor pressure of air (VP SatAir ). The vapor pressure (VP) of air can be estimated from Eq. (5) as follows: where RH is the relative humidity of air in %.
Trend analysis. As a part of the trend analysis, the modified Mann-Kendall test 28 , Pettitt test 29 , and Sen's slope 30 were performed using R software for each time series. All the tests were carried out at a level of significance less than 5% probability (p ≤ 0.05). Together, Mann-Kendall and Sen's slope tests have been used in several hydro climatological studies [31][32][33] . The growing season average of wet bulb depression and vapor pressure deficit VPD (one average value for the growing season in the calendar year) and averages of temperature, relative humidity, wind speed and wet bulb depression and VPD at critical pesticide application stages for cotton ( Table 1) were used. It should be noted that there were nine distinct pesticide application periods identified for cotton cultivation in the study area. However, periods four and five were only one day before/after (Table 1). Therefore, from the perspective of baseline vs. climate change analysis, periods four and five were combined, which resulted in eight periods for trend analysis results. The pesticide application details described in the manuscript are based on a summary of the commonly used land management practices for cotton in the region. We have not conducted any field experiment(s) as a part of this study.
Modified Mann-Kendall test. The Mann-Kendall test is a nonparametric test to investigate the existence of any monotonous increasing or decreasing trends in data. Most weather data will have autocorrelation (dependency of one data entry on the previous one, for example, dependence of today's relative humidity on the previous day's relative humidity). This type of autocorrelation might interfere with the clear identification of trends in the data. Therefore, the modified Mann-Kendall test that addresses the autocorrelation in the data were used in this study. The modified Mann-Kendall test was performed using the package 'modifiedmk' developed for use in R.
Pettitt test. The nonparametric Pettitt test is used to identify the single change point in the continuous time series of data. The Pettitt test is based on the rank-based Mann-Whitney test 34 . It has been used in several climatological studies [35][36][37] . The 'trend' package available in R was used to perform the Pettitt test.
Sen's slope. Sen's slope test is used to estimate the magnitude of the trend (linear rate of change) in the data. Both the slope and the intercept are computed as a part of this test. Sen's slope is a median parameter estimated (5) VP air = VP SatAir × RH Table 1. Timing and type of chemical application for cotton grown in Stoneville, MS (the window of dates is selected based on three years of field data to accommodate year-to-year changes in dates of chemical application).  www.nature.com/scientificreports/ from the series of linear slopes estimated between sets of two data points. The test provides 95% confidence limits of results as well. It was performed using the package 'trend' in R. How are the results analyzed? To analyze the results, statistical significance was used. Statistical significance indicates whether the results obtained are very likely or could have been obtained by chance. In the context of trend analysis, it measures the probability of the null hypothesis becoming true with a predefined significance level. In this study, a 5% significance level is used to judge the results. For example, suppose the results show a decreasing trend in relative humidity and are statistically significant; in that case, the relative humidity declines estimated by trend analysis is very likely, and the results are statistically defensible. If the probability values are relatively higher than the 5% level, the results are less reliable, and there is uncertainty around the results. In the manuscript, this item is categorized as less significant. If the trend analysis results are not statistically significant, it indicates that the result could have occurred by chance, and they are not reliable.

Results
Changes in seasonal average weather parameters. The trend analysis results of the magnitude of the seasonal average and extreme parameters are presented in Table 2. The corresponding statistical significance of the results is presented in Table 3. There is a monotonous increase in the seasonal average air temperature. The  (Table 2). Similarly, there is an increase in relative humidity and an increase in wind speed, the details of which are presented in Table 2. All the trend analysis results are statistically significant for average temperature, average relative humidity, and minimum temperature. None of the trend analysis results are significant for minimum wind speed. For all the other parameters, some tests showed statistical significance, and others showed no significance (Table 3).
Qualitative changes in pesticide vapor drift. Trends in wet bulb depression (estimated from the air temperature and relative humidity) indicative of evaporative demand and vapor pressure deficit indicative of room in air for pesticide vapors are shown in Fig. 2. Decreasing trends are noticed for the evaporative demand, and the results are statistically significant for the modified Mann-Kendall test, less significant for Sen's slope and not significant for the Pettitt test. Conversely, increasing trends are observed for VPD, although the numerical statistics indicate otherwise. However, none of the VPD results are statistically significant. The volatilization of the applied pesticide is a function of the pesticide, atomization pattern, and weather parameters. Given the scenario of the same chemical application using similar atomization methods each year, any changes in weather parameters will likely alter the volatilization of the particular chemical. The decrease in evaporative demand (as shown in Fig. 2a) indicates decreased (climate change-induced) volatilization of the pesticide in the LMD. The increased VPD indicates a higher possibility for volatilization, contradicting the results of WBD or evaporative demand. Although both VPD and WBD use both the temperature and relative humidity of air, there are some differences in the way they are mathematically used to derive the respective parameters. VPD shows the dryness of air or the capacity to accept additional vapors whereas WBD is influenced by humidity. Depending on atmospheric conditions, both WBD and VPD can provide different insights into the evaporative conditions of the atmosphere. Moreover, the information provided in Fig. 2 is based on seasonal averages alone. Analyzing similar details on each pesticide application period using historic weather data will shed more light on climate-induced pesticide volatilization patterns. www.nature.com/scientificreports/ Climate change during critical pesticide application periods within the cotton growing season. The trend analysis results of weather parameters at important stages of pesticide application during cotton growth are presented in Fig. 3 and Table 4. The corresponding statistical significance of the results is presented in Fig. 4. During the preemergent herbicide application and the two insecticide applications during the beginning of summer, there was a temperature decline (Fig. 3a) for the climate change scenario when compared to the baseline. However, during late spring and most of the later part of summer, there is a consistent increase in air temperature during pesticide applications for cotton in the climate change scenario. Unlike temperature, there was a decline in RH throughout the growing season between baseline and altered conditions, except during the third pesticide application period. The difference in RH between the baseline and altered climate conditions was as much as 5% for pesticide application during the later part of the summer. Although the seasonal average and seasonal extreme wind speed data showed some trends, within-season values did not show any noticeable trend except for the preemergent herbicide application (first pesticide application of the season). The trend analysis results of weather parameters are most significant for relative humidity, less significant for temperature and almost insignificant for wind speed (Fig. 4). Therefore, the cotton-applied pesticide vapor drift pertaining to climate change in LMD is limited to the combined effect of changes in temperature (most increases and sometimes declines) and relative humidity alone, which is shown in Fig. 3d,e. Given the same chemical and application rates using the same equipment year after www.nature.com/scientificreports/ year, climate change that occurred in the past in the LMD could have increased the pesticide vapor drift/pesticide volatilization during most of the cotton growing season (except application period three), as evidenced by Fig. 3d showing wet bulb depressions. In terms of proportional increases (in climate change period than baseline), the wet bulb depressions range from − 13 to 21%. However, Fig. 3e shows consistent increases in VPD, suggesting increases in vapor drift/pesticide volatilization throughout the cotton growing season. The increased proportions during the climate-altered period range from 12 to 43%.

Discussion
Increasing air temperature during the cotton growing season has been highlighted in several previous studies 38,39 . Similarly, changes in relative humidity during the crop growth period are also documented 40 . The volatilization of applied pesticides is a function of the physico-chemical characteristics (especially vapor pressure) of pesticides 13 , plant ability to absorb chemicals, soil pH 41 , application equipment (the way pesticides are atomized) 18 and weather parameters 13 , especially air temperature, relative humidity, and wind speed 7 . Leaf area, proportion of pesticide intercepted by canopy, and leaf height were also reported to affect pesticide volatilization 11 . Similar to this study, higher pesticide volatilization under climate change is reported in Delcour et al. 42 and Tudi et al. 43 . Similar to this study, the analysis by Ferraro and de Paula 18 describes wet bulb depression as one of the parameters determining pesticide volatilization. In this study, both the wet bulb depression and VPD were used to arrive at conclusions on possible climate change effects on pesticide volatilization. There are, however, differences between the results of WBD and VPD. Based on the physico-chemical characteristics of the pesticides applied for cotton in LMD (Table 5), especially the vapor pressure and Henry's law constant 44 , it appears that pendimethalin applied in period 1 and S-metolachlor applied during period 4 show more volatility than other pesticides. The increasing trends indicated by both WBD and VPD during period 1 are statistically insignificant. Therefore, www.nature.com/scientificreports/ pendimethalin applied during period 1 may not have volatilized more because of climate change. However, period 4 pesticide application corresponding to the S-metolachlor application shows increasing trends both by WBD and by VPD (Table 4, Figs. 3 and 4) that are statistically significant. Therefore, S-metolachlor applied in cotton during application period 4 could have volatilized more in LMD than any other pesticide on the list. It should be noted that this conclusion is based on the physico-chemical properties of pesticides and trend analysis of weather and weather-derived parameters. Field data collection and analysis are needed to verify this result. Apart from climate change, other factors could be responsible for the increased volatilization of pesticides. Higher pesticide volatilization from moist soils than dry soils is documented by Schneider and Goss 47 . A dry soil will have plenty of surface area for the pesticide to adsorb. However, rainfall or irrigation before or after application will significantly reduce the surface available for the pesticide to adsorb and become readily available for volatilization 47 . This could also be viewed from another dimension of timing decisions on pesticide application. For example, targeting the pesticide application date prior to a few dry days and avoiding times with high heat will help to minimize volatilization problems. This can be accomplished by having access to a reliable weather forecast for a week. Other factors, such as human decisions on spraying equipment selection (type and number of nozzles), application method (aerial vs. ground), and respecting or not respecting the application instructions recommended in the pesticide label, could also affect pesticide volatilization. Ensuring the compliance of the other factors mentioned here is a prerequisite to pinpointing the role of climate change alone in increasing pesticide volatilization and vapor drift.
Limitations of the study. To interpret the results, wet bulb depression, which is a function of temperature and relative humidity, was used in this study. Although wind speed is certainly a factor influencing volatilization, it was not included in the analysis because the extent of changes in wind speed during the climate change scenario (when compared to baseline) is negligible and the trends were not statistically significant. However, subdaily or instantaneous wind speeds for different scenarios could have provided some useful information. However, they were not available. This could have some errors associated with the analysis results. To estimate www.nature.com/scientificreports/ the vapor pressure of the pesticide evaporating surface (leaf/soil), air temperature is used instead of leaf/soil temperature. For our analysis, long-term records of leaf/soil temperatures are not available. Therefore, air temperature is used for computations. However, in general, plants without water stress will have leaf temperatures cooler than the air temperature. Additionally, soil temperatures vary from air temperature. These could have introduced errors in the results. This study attempted to quantify only the volatilization part of the vapor drift.
The other important part of vertical mixing of the evaporated pesticide vapors in the atmosphere, its degradation in the atmosphere and the subsequent transport in the downwind direction are not addressed. However, they could be pursued as a future effort.

Summary and conclusions
In this study, an attempt was made to ascertain the possible consequences of climate change on pesticide vapor drift using data on pesticide applications to cotton in Stoneville, MS, USA. Pesticide vapor drift is a two-step process: (a) volatilization of the applied pesticide to vapors and (b) mixing of the vapors with the atmosphere and Table 4. Trend analysis results on the magnitude of average weather parameters during critical herbicide, insecticide, and growth regulator application periods. www.nature.com/scientificreports/ their transport in the downwind direction. This study dealt with the volatilization part alone. Fifty-five years of daily weather data from 1960 to 2014 were used for the trend analysis. Patterns in wet bulb depression (WBD) and vapor pressure deficit (VPD) estimated from weather data were related to pesticide volatilization potential. Based on the results obtained, we can conclude the following: a. Significant changes occurred in the seasonal average daily temperature and seasonal average relative humidity. b. Noticeable changes occurred in daily average temperature, and significant changes were noticeable in the daily average relative humidity during critical stages of pesticide application for cotton. c. Based on the physico-chemical characteristics of pesticides and trend analysis results of weather, it appears that the S-metolachlor applied in cotton during the middle of July could have volatilized more in the study area than any other pesticide. d. In summary, the changes in temperature and declines in relative humidity during the cotton growing season could have increased the pesticide volatilization/vapor drift estimated from WBD and VPD in the region.

Data availability
Data used in the study are available on request.